[en] The low-temperature phase transition (-22.4 deg. C) is accompanied by marked change of the optical properties, as was first revealed for 3-phenylthiopentan-2,4-dionatoboron difluoride (1) in the series of β-diketonatoboron difluorides. This transition was investigated by luminescence and reflection spectroscopy, differential scanning calorimetry (DSL), X-ray analysis and atomic force microscopy methods. It was observed by the DSL method that at the room temperature the complex existed in its high-temperature modification and transformed into the stable one with ΔH=0.981 kcal/mol below -22.4 deg. C. The reverse transition takes place at 43.4 deg. C with ΔH=0.59 kcal/mol. Cooling of 1 results in a noticeable change of its morphology: the crystal cracked along the layer borders and nanometer particles were formed. Having been cooled the light-yellow crystals turned into white powder and luminescence colour changed from aquamarine to white. Heating of the cooled 1 to the 47.9 deg. C (the second phase transition) results in partial coincidence of the reflection and luminescence spectra, as well as those of initial single crystals that indicates the existence of size-dependent optic properties